This proposal is aimed at establishing neural sites and cellular mechanisms by which gonadal hormones promote sex differences in vocal ability and neural organization in passerine birds. In zebra finches, song is an androgen-dependent behavior that is normally produced only by males. Estrogens act during development to determine the organization, and, importantly, the extent androgen accumulation within song regions. Androgens induce further neural growth in adulthood, and promote song development. This proposal focuses on three important issues regarding the secual differentiation of the song system. The first regards where estrogen (E2) acts to masculinize developing regions. Steroid autoradiography will be used to assess which developing song region accumulate E2. Intracranial E2 implants will be used to determine which of these neural sites are primary targets for E2 action, and which are influenced indirectly, or trans-synaptically. The second issue concerns how early E2 exposure determines the extent of androgen accumulation and sensitivity in the adult brain. Steroid autoradiography will be used to assess androgen accumulation within developing song regions and hence, whether E2 induces, or preserves a masculine pattern of androgen accumulation. The existence of a critical period for this aspect of E2 action will be determine by examining the pattern of androgen accumulation in birds given systemic E2 in adulthood. Thymidine autoradiography will be used to determine if neurogenesis occurs during the critical period of E2 action, and if E2 stimulates this neurogenesis. The third issue regards the neural sites and functional significance of androgen action in the adult brain. Using light microscopic analyses of cell morphology in song regions, and behavioral analyses, the timecourse of androgen-induced neural change will be assessed and related to vocal development. Projections of androgen-accumulating cells will be established throught the simultaneous use of fluorescent retrograde tracers and steroid autoradiography, and intracranial antiandrogen implants will be used to determine if androgen-accumulating neurons in certain song regions mediate androgen-induced neural growth in their efferent targets. A better understanding of the cellular mechanisms of sexual differentiation hold great promise for therapeutic remedy of abnormalities in sexual differentiation in humans. It will also increase our understanding of cellular mechanisms that permit aspects of neural plasticity that are manifest in birds, yet less robust in humans.